Optical spectra of strong-coupling polarons

The photodissociation spectra of Landau-Pekar polarons are calculated using the theory of quantum coherent states. It is shown that the number of phonons emitted in one dissociation event can differ and that their energy is equal to the doubled polaron binding energy E _p only on the average. It is established that the absorption spectrum is a superposition of bands corresponding to different numbers of phonons emitted during the dissociation of one polaron and that the half-width of each of the bands is much greater than the distance between the bands (which is equal to the phonon energy ?θ). Therefore, the absorption spectrum looks like a very wide unstructured band with the low-frequency edge lying at E _p + ?θ, a maximum at an energy of about 5E _p (for the band carrier mass equal to (1–3)m _e ), and the half-width being of the order of the energy corresponding to the maximum.     

Clean TROSY: compensation for relaxation-induced artifacts

TROSY pulse sequences for recording, e.g., (1)H-(15)N chemical shift correlation spectra of proteins are designed to select only one of four two-dimensional multiplet components. However, all of the variants published so far are prone to relaxation-induced artifacts at the positions of two of the other multiplet components. This article introduces modifications to the two spin-state-selective coherence transfer building blocks of the TROSY mixing sequence resulting in a clean TROSY spectrum with the artifacts largely suppressed. It works by having the new mixing sequence generate peaks of opposite phase at the positions of the relaxation artifacts. The clean TROSY pulse sequence is marginally shorter than the original one and contains the same pulses. Experimental demonstration is presented for the (15)N-labeled proteins RAP 17-97 (N-terminal domain of alpha(2)-macroglobulin receptor associated protein) and EQT, equinatoxin II, from the Mediterranean anemone Actinia equina.     

HAT HMBC: a hybrid of H2BC and HMBC overcoming shortcomings of both

A new 2D NMR experiment, HAT HMBC, that is a hybrid of H2BC and HMBC aims at establishing two-bond correlations absent in H2BC spectra because of vanishing (3)J(HH) coupling constants. The basic idea is to create an additional pi phase difference in the multiplet structure in HMBC peaks with respect to the (n+1)J(HH) coupling constant between the proton(s) attached to a (13)C and a (1)H separated by n bonds. Thus HMBC peaks associated with small J(HH) will be the most attenuated in a HAT HMBC spectrum in comparison to a regular HMBC spectrum, i.e. peaks associated with (n+1)J(HH) and (n)J(CH) will for n>2 usually be strongly attenuated. The HAT HMBC pulse sequences contain the same number of pulses as regular HMBC and are only a few milliseconds longer.     

External removal of endpoint-discontinuity artifacts in the reciprocal-space analysis of spectra

We present a systematic method of removing endpoint-discontinuity artifacts in the Fourier analysis of spectral segments, enabling the more accurate extraction of information. This principal-component-removal approach differs from a previous version by using extrapolated (or extended) data outside rather than inside the spectral range. This not only allows coefficients to be accessed to the white-noise limit with no distortion of the segment, but also generates interpolated coefficients for improved analytic insight. Examples are provided.     

Scattering noise and measurement artifacts in a single-shot cross-correlator and their suppression

We studied the optical scattering noise, measurement artifacts, and their suppression in single-shot measurements of prepulse contrast. The measurement noise due to air scattering and the two kinds of measurement artifacts resulting from multiple reflections of the generated correlation signal and correlation between the pulse under test and the internally reflected sampling pulse were experimentally explored. The scattering noise and measurement artifacts significantly degrade the measurement fidelity. We demonstrate the clean measurement of prepulse contrast in a single-shot cross-correlator by attenuating the main peak of the correlation beam with a dot mirror and designing the correlating process based on a periodically-poled lithium niobate crystal embedded in an unpoled wafer. The standard time-scanning measurements of pulse contrast are also performed to confirm the experimental results of our single-shot measurement.     

Peak suppression in ESEEM spectra of multinuclear spin systems

We have observed a disturbing suppression effect in three-pulse ESEEM and HYSCORE spectra of systems with more than one nucleus coupled to the electron spin. For such systems, the ESEEM signal contains internuclear combination peaks of varying intensity. At the same time, the peaks at the basic ESEEM frequencies are reduced in intensity, up to the point of complete cancellation. For both three-pulse ESEEM and HYSCORE, the amplitude of a peak of a given nucleus depends not only on its modulation depth parameter k and the tau-dependent blind-spot term b, but also on k and b of all other nuclei. Peaks of nuclei with shallow modulations can be strongly suppressed by nuclei with deep modulations. This cross-suppression effect explains the observation that HYSCORE (1)H peaks are often very weak or even undetectable in the presence of strong (14)N peaks. Due to this distortion of intensities, ESEEM spectra have to be analysed very carefully. We present a theoretical analysis of this effect based on the product rules, numerical computations, and illustrative experimental data on Cu(gly)(2). In experiments, the impact of this cross suppression can be alleviated by a proper choice of tau values, remote echo detection, and matched pulses.     

Development of a method for the measurement of long-range 13C-1H coupling constants from HMBC spectra

This paper describes a number of improvements to a method, developed in this laboratory and described in J. Magn. Reson. 85 (1989) 111-113, which makes it possible to determine values of long-range 13C-1H coupling constants from heteronuclear multiple bond correlation (HMBC) spectra. First, it is shown how pulsed-field gradients can be introduced into the HMBC experiment without perturbing the form of the cross-peak multiplets; a one-dimensional version of the experiment is also described which permits the rapid measurement of a small number of couplings. Second, the experiment is modified so that one-bond and long-range cross-peaks can be separated, and so that the one-bond cross-peaks have more reliable intensities. Finally, it is shown how these one-bond cross-peaks can be used to advantage in the fitting procedure.     

Artificial neural network for suppression of banding artifacts in balanced steady-state free precession MRI

The balanced steady-state free precession (bSSFP) MR sequence is frequently used in clinics, but is sensitive to off-resonance effects, which can cause banding artifacts. Often multiple bSSFP datasets are acquired at different phase cycling (PC) angles and then combined in a special way for banding artifact suppression. Many strategies of combining the datasets have been suggested for banding artifact suppression, but there are still limitations in their performance, especially when the number of phase-cycled bSSFP datasets is small. The purpose of this study is to develop a learning-based model to combine the multiple phase-cycled bSSFP datasets for better banding artifact suppression. Multilayer perceptron (MLP) is a feedforward artificial neural network consisting of three layers of input, hidden, and output layers. MLP models were trained by input bSSFP datasets acquired from human brain and knee at 3T, which were separately performed for two and four PC angles. Banding-free bSSFP images were generated by maximum-intensity projection (MIP) of 8 or 12 phase-cycled datasets and were used as targets for training the output layer. The trained MLP models were applied to another brain and knee datasets acquired with different scan parameters and also to multiple phase-cycled bSSFP functional MRI datasets acquired on rat brain at 9.4T, in comparison with the conventional MIP method. Simulations were also performed to validate the MLP approach. Both the simulations and human experiments demonstrated that MLP suppressed banding artifacts significantly, superior to MIP in both banding artifact suppression and SNR efficiency. MLP demonstrated superior performance over MIP for the 9.4T fMRI data as well, which was not used for training the models, while visually preserving the fMRI maps very well. Artificial neural network is a promising technique for combining multiple phase-cycled bSSFP datasets for banding artifact suppression.     

Reduction of spin diffusion artifacts from 2D zfr-INADEQUATE MAS NMR spectra

The primary shortcoming of the z-filtered refocused INADEQUATE MAS NMR pulse sequence is the possibility of artifacts introduced during the z-filter due to spin diffusion where by extra peaks in the single-quantum dimension (from other sites in the molecule) appear correlated with a given double-quantum frequency. This is a problem when the spinning speeds are too slow (less than 15 kHz) to sufficiently average the proton-proton homonuclear dipolar couplings. This would be especially important when working with large volume rotors that are difficult to spin fast enough to completely average the homonuclear couplings. In our experiments we used the frequency-switched Lee-Goldberg (FSLG) method of homonuclear decoupling during the z-filter to remove the artifact peaks. This method has the advantage of being quite easy to setup and implement on most modern NMR spectrometers.     

Curves of marginal stability, and weak- and strong-coupling BPS spectra in N = 2 supersymmetric QCD

We explicitly determine the global structure of the SL(2, ) bundle over the Coulomb branch of the moduli space of asymptotically free N = 2 supersymmetric Yang-Mills theories with gauge group SU(2) when massless hypermultiplets are present. For each relevant number of flavours, we show that there is a curve of marginal stability of the Coulomb branch, diffeomorphic to a circle, across which the BPS spectrum is discontinuous. We determine rigorously and completely the BPS spectra inside and outside the curve. In all cases, the spectrum inside the curve consists of only those BPS states that are responsible for the singularities of the low energy effective action (in addition to the massless abelian gauge multiplet which is always present). The predicted decay patterns across the curve of marginal stability are perfectly consistent with all quantum numbers carried by the BPS states. As a byproduct, we also show that the electric and magnetic quantum numbers of the massless states at the singularities proposed by Seiberg and Witten are the only possible ones.     

Properties of strong-coupling magnetopolaron in quantum rods

The Hamiltonian of a quantum rod with an ellipsoidal boundary is given after a coordinate transformation that changes the ellipsoidal boundary into a spherical one. We then study the vibrational frequency and the ground state binding energy of the strong-coupling magnetopolaron in a quantum rod. The effects of the electron–phonon coupling strength, the cyclotron frequency of a magnetic field, the aspect ratio of the ellipsoid and the transverse and longitudinal effective confinement lengths are taken into consideration by using the linear combination operator method. It is found that the vibrational frequency and the ground state binding energy are increasing functions of the electron–phonon coupling strength and the magnetic field cyclotron frequency, whereas they are decreasing functions of the aspect ratio of the ellipsoid and effective confinement lengths.     

WAVEWAT-improved solvent suppression in NMR spectra employing wavelet transforms

WAVEWAT is a new processing algorithm to suppress the on-resonance water signal in NMR spectra. It is based on a multiresolution analysis (MRA) of the free induction decay (FID) using a dyadic discrete wavelet transform (DWT). The width of the suppressed signal can be adjusted so that signals close to water are recovered without distortion of the signal shape and intensity. Computational efficiency is comparable to that of convolution filters employing a Fourier transform.     

Randomized acquisition for the suppression of systematic F1 artifacts in two-dimensional NMR spectroscopy.

2D spectra, particularly for homonuclear correlation, can show a variety of artifactual signals in the F1 domain. Common sources include carry-over of signal modulation from one transient to the next ("rapid pulsing artifacts") and systematic variations in room temperature ("parallel diagonals"). In both cases there is one very simple expedient which can greatly reduce the impact of these sources of error. Multidimensional data sets are almost invariably recorded by simply incrementing or decrementing evolution periods, largely for reasons of convenience and historical precedent. If instead the sampling of the evolution periods is carried out in random order, the perturbations responsible for the sharp F1 signals in the conventional experiment manifest themselves as t1 noise. Since the randomized acquisition redistributes coherent artifactual signals randomly in F1, the maximum artifactual signal is substantially reduced in the randomized experiment and no longer appears in the form of misleading distinct peaks.     

Rashba自旋-轨道相互作用影响下量子盘中强耦合磁极化子性质的研究

本文基于Lee-Low-Pines幺正变换法,采用Tokuda改进的线性组合算符法研究了Rashba自旋-轨道相互作用效应下量子盘中强耦合磁极化子的性质.结果表明,磁极化子的相互作用能Eint的取值随量子盘横向受限强度ω0、外磁场的回旋频率ωc、电子-LO声子耦合强度α和量子盘厚度L的变化均与磁极化子的状态性质密切相关;磁极化子的平均声子数N随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在Rashba自旋-轨道相互作用效应影响下磁极化子的有效质量将劈裂为m*+,m*-两种,它们随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在研究量子盘中磁极化子问题时,电子-LO声子耦合和Rashba自旋-轨道相互作用效应的影响不可忽略,但Rashba自旋-轨道相互作用和极化子效应对磁极化子的影响只有在电子运动的速率较慢时显著.     

Attraction of like-charged macroions in the strong-coupling limit

Like-charged macroions attract each other as a result of strong electrostatic correlations in the presence of multivalent counterions or at low temperatures. We investigate the effective electrostatic interaction between i) two like-charged rods and ii) two like-charged spheres using the recently introduced strong-coupling theory, which becomes asymptotically exact in the limit of large coupling parameter (i.e. for large counterion valency, low temperature, or high surface charge density on macroions). In contrast to previous applications of the strong-coupling theory, we deal with curved surfaces and an additional parameter, referred to as Manning parameter, is introduced, which measures the ratio between the radius of curvature of macroions to the Gouy-Chapman length. This parameter, together with the size of the confining box enclosing the two macroions and their neutralizing counterions, controls the counterion-condensation process that directly affects the effective interactions. For sufficiently large Manning parameters (weakly-curved surfaces), we find a strong long-ranged attraction between two macroions that form a closely-packed bound state with small surface-to-surface separation of the order of the counterion diameter in agreement with recent simulations results. For small Manning parameters (highly-curved surfaces), on the other hand, the equilibrium separation increases and the macroions unbind from each other as the confinement volume increases to infinity. This occurs via a continuous universal unbinding transition for two charged rods at a threshold Manning parameter of , while the transition is strongly discontinuous for spheres because of a pronounced potential barrier at intermediate distances. Unlike the cylindrical case, the attractive forces between spheres disappear slowly for increasing confinement volume due to the complete de-condensation of counterions. Scaling arguments suggest that for moderate values of coupling parameter, strong-coupling predictions remain valid for sufficiently small surface-to-surface separations.Received: 20 August 2003, Published online: 2 March 2004PACS: 87.15.-v Biomolecules: structure and physical properties - 82.70.Dd Colloids - 87.15.Nn Properties of solutions; aggregation and crystallization of macromolecules     

Gradient induced artifacts in simultaneous EEG-fMRI: Effect of synchronization on spiral and EPI k-space trajectories

The nature of the gradient induced electroencephalography (EEG) artifact is analyzed and compared for two functional magnetic resonance imaging (fMRI) pulse sequences with different k-space trajectories: echo planar imaging (EPI) and spiral. Furthermore, the performance of the average artifact subtraction algorithm (AAS) to remove the gradient artifact for both sequences is evaluated. The results show that the EEG gradient artifact for spiral sequences is one order of magnitude higher than for EPI sequences due to the chirping spectrum of the spiral sequence and the dB/dt of its crusher gradients. However, in the presence of accurate synchronization, the use of AAS yields the same artifact suppression efficiency for both pulse sequences below 80 Hz. The quality of EEG signal after AAS is demonstrated for phantom and human data. EEG spectrogram and visual evoked potential (VEP) are compared outside the scanner and use both EPI and spiral pulse sequences. MR related artifact residues affect the spectra over 40 Hz (less than 0.2 μV up to 120 Hz) and modify the amplitude of P1, N2 and P300 in the VEP. These modifications in the EEG signal have to be taken into account when interpreting EEG data acquired in simultaneous EEG-fMRI experiments.     

Transmission induced structure in electron emission spectra

Angle resolved secondary electron emission spectra from tungsten contain fine-structure which to a large extent can be correlated with high density-of-states regions in the bulk band structure. In addition to such structure, strong “extra” peaks have been observed. The “anomalous” peaks, which are particularly dominating in the W{111}-spectra, occur when the “emitting” bulk Bloch state has a group velocity that equals the group velocity of the emitted electron. These particular peaks reflect structure in the transmission probability function and can only be explained by considering the wave-matching at the surface.     

Electromagnetically induced tunnelling suppression in a flux qubit

Motivated by recent experiments wherein Josephson devices are irradiated by microwaves fields or are coupled to LC resonators, we theoretically investigate the dynamics of a flux qubit coupled to a monochromatic bosonic mode. We define strong coupling conditions under which the qubit tunnelling frequency between the localized flux states can be controlled and even suppressed. The practical realization of such a regime leading to this hindered dynamics is discussed.Received: 21 May 2003, Published online: 11 August 2003PACS: 03.67-a Quantum information - 03.67.Lx Quantum computing - 85.25.Dq Superconducting quantum interference devices (SQUIDs)